1. Field of the Invention
The present invention relates generally to spacecraft propulsion systems and, more particularly, to electrostatic propulsion systems.
2. Description of the Related Art
On-board propulsion systems are used to realize a variety of spacecraft maneuvers such as orbit raising (e.g., raising from a low Earth orbit to a geostationary orbit), stationkeeping (e.g., correcting the inclination, drift and eccentricity of a satellite's orbit) and attitude control (e.g., correcting attitude errors about a satellite's roll, pitch and yaw axes).
The force exerted on a spacecraft by a propulsion system's thruster is expressed in equation (1) ##EQU1## as the product of the thruster's mass flow rate and the thruster's exhaust velocity. Equation (1) also shows that mass flow rate can be replaced by the ratio of weight flow rate to the acceleration of gravity and that the ratio of exhaust velocity to the acceleration of gravity can be represented by specific impulse I.sub.sp which is a thruster figure of merit. Equation (1) can be rewritten as equation (2) ##EQU2## to show that specific impulse is the ratio of thrust to weight flow rate.
When a thruster is used to effect a spacecraft maneuver, a velocity increase .increment.V of the spacecraft is gained with a differential loss in mass of stored fuel, i.e., a differential between the spacecraft's initial mass M.sub.i (prior to the maneuver) and the spacecraft's final mass M.sub.f (after the maneuver). This mass differential is a function of the thruster's specific impulse I.sub.sp as expressed by the "rocket equation" of ##EQU3## in which .increment.V has units of meters/second, I.sub.sp has units of seconds and a constant g is the acceleration of gravity in meter/second.sup.2. Equation (3) states that fuel loss causes a spacecraft's final mass M.sub.f to exponentially decrease with increased .increment.V and that this decrease can be exponentially offset by an increase in specific impulse I.sub.sp.
Specific impulse is therefore an important measure of a thruster's fuel efficiency. Basic thruster types and their specific impulses are described in various spacecraft sources (e.g., Morgan, Walter L., et al., Communications Satellite Handbook, John Wiley and Sons, New York, 1989, pp. 651-653, 656, 657, 849, 850 and 867-869). Typical specific impulses are 230 seconds for monopropellant thrusters, 290 seconds for solid propellant thrusters, 445 seconds for bipropellant thrusters and 500 seconds for electric arc jet thrusters.
In contrast to these thruster types, electrostatic thrusters achieve thrust through the interaction of electrostatic fields on charged propellant particles such as ions and charged colloids. Conventional electrostatic thrusters (as described, for example, in Sutton, George F., Rocket Propulsion Elements, John Wiley and Sons, New York, 1992, pp. 580-590) include electron bombardment thrusters, ion contact thrusters and field emission or colloid thrusters. In electron bombardment thrusters, ions are produced by bombarding a gaseous propellant (e.g., xenon) with electrons from a heated cathode. In ion contact thrusters, ions are produced by passing a propellant vapor through a hot, porous contact ionizer (made, for example, of tungsten). In field emission thrusters, propellant droplets are electrically charged by passing them through intense electric fields. Electrostatic thrusters are capable of very high specific impulses (e.g., ion thrusters have been developed with specific impulses in excess of 2500 seconds).
The high specific impulse of electrostatic thrusters makes them an attractive thruster for spacecraft maneuvers. Their high fuel efficiency can facilitate a reduction of initial satellite mass, an increased payload and a longer on-orbit lifetime. Reduction of initial mass lowers the spacecraft's initial launch cost and increased payload and longer lifetime increase the revenue that is generated by the spacecraft.
Spacecraft generally carry a plurality of thrusters to effect spacecraft maneuvers. Each of these thrusters has typically been provided with its own power supply system (e.g., a spacecraft with four thrusters also has four thruster power supply systems). Because weight and volume are at a premium in spacecraft, a simpler propulsion system is desirable.